Virus that makes humans stupid discovered.

Scientists stumbled on something unexpected while cataloguing the microbes in human throats: DNA from a virus previously thought to infect only green algae — and the people carrying it scored modestly lower on tests of thinking and attention.

The virus is ATCV-1 (Acanthocystis turfacea chlorella virus 1), common in freshwater lakes and rivers. Researchers at Johns Hopkins and the University of Nebraska–Lincoln found its DNA in throat swabs from 40 of 92 healthy participants in a cognitive-function study — about 43 percent. Those who carried it performed significantly worse on tests of visual processing and visual-motor speed, scoring on average seven to nine points lower than the uninfected group (PNAS abstract, PMID 25349393).

The team also tested mice: those exposed to ATCV-1 took longer to explore a novel maze and showed poorer recognition memory and sensory-motor gating, along with altered gene expression in the hippocampus — genes tied to synaptic plasticity, learning and memory.

"This is a striking example showing that the 'innocuous' microorganisms we carry can affect behavior and cognition," lead investigator Robert Yolken, a virologist at Johns Hopkins, told The Independent. The researchers stressed that causation had not been proven and that ATCV-1 should not be treated as a threat to public health — but the finding opened a new question about how the invisible organisms we harbor shape the way we think.

Read more: The Independent's 2014 report